1. The Field of the Invention
The present invention relates to spectrometers, and particularly to X-ray fluorescence spectrometers for wear metal analysis of lubricating oils.
2. Relevant Technology
The presence of wear metal particles in lubricating oils, even in micron or sub-micron size is recognized as one of the main causes of catastrophic failure of aircraft engines, gear boxes, and other lubricated machinery. The American Society of Mechanical Engineers ("ASME") has established standard ASME industrial assessment charts such as the one depicted in FIG. 1, indicating industrial tolerance limits for various chemical elements. Some types of equipment require even more stringent standards. For example, advanced military aircraft engines, have a limit for the maximum iron content for safe engine operation that is typically set at about four (4) parts per million ("ppm"). This is a much lower limit than the range of 20 ppm to 50 ppm established by one ASME industrial assessment chart for general industrial facilities as indicated by FIG. 1.
Depending on the type of equipment, the risk of failure, and the stringentness of the tolerance limits, periodic inspections are performed to verify that the wear metal particles are not exceeding established tolerances. For example, after a number of in-flight occurrences resulting in the loss of crew and hardware, the risk of engine failure was deemed to be so serious by the military that an oil analysis for F16 fighter airplanes is currently performed after every flight.
At present, wear metal analysis is routinely performed in dedicated analytical laboratories on solid residual particles collected from used oils by filtration after selected periods of operation, such as 10, 50, 100, and 500 hours of operation. The most common analysis methods are: atomic emission spectroscopy; inductively coupled plasma spectroscopy; atomic absorption spectroscopy; and X-ray fluorescence spectroscopy. Atomic emission spectroscopy is a destructive method in which a sample material is excited by arc sparks that are produced in a sample chamber to cause a subsequent characteristic emission. Inductively coupled plasma spectroscopy is also a destructive method. In inductively coupled plasma spectroscopy a sample is excited by burning it in a high temperature furnace to produce a plasma. Atomic absorption spectroscopy is a destructive inspection method in which the molecular disassociation or ionization of the sample is related to characteristic reductions in the intensity of an incident light beam passing through the sample. The X-ray fluorescence spectroscopy is a non-destructive inspection method where a sample is excited by exposure to an X-ray beam and as the energy dissipates releases a characteristic emission.
In certain field cases, periodic batch sampling and emission or absorption analyses are sufficiently reliable to detect over ninety percent (90%) of potential failures. This level of performance is, however, considered insufficient or unacceptable if, even in singular cases, loss of human life and of unique, expensive hardware is involved. Moreover, emission and absorption spectroscopies are sensitive only to solid particles of a size in the 1 micron to 10 micron range. Thus, these methods cannot be used for advanced engine systems in which special filters prevent the passage of particles larger than 0.3 microns.
A particular deficiency of the inductively coupled plasma spectroscopy is that the short, high temperature burning of the batch specimens usually lasting only nanoseconds to microseconds may not be capable of vaporizing large wear metal particles. These limitations of absorption and emission spectroscopies suggest that X-ray fluorescence spectroscopy should rather be adopted as a preferred analytical method.
X-ray fluorescence spectroscopy has no particle size limitation, and is fast, reliable, and non-destructive. In current applications, however, X-ray fluorescence spectroscopy is still performed in the batch mode. A batch mode is when the sample materials, in this case the solid particles, are collected periodically from used oils by a filtration process. Therefore, in spite of the advantages of the X-ray fluorescence spectroscopy method, the formation and release of wear metal particles into the system may still escape immediate detection with possible catastrophic results. On the other hand, oil analyses performed with virtually arbitrary periodicity, without any reliable indication of actual significant wear, are expensive in terms of materials, equipment, labor, and time.
X-ray fluorescence spectroscopy is one of the analytical methods often used for elemental identification and quantitative evaluation of components in multi-element sample materials. A schematic illustration of the basic two-stage process involved in X-ray fluorescence spectroscopy is shown in FIGS. 2 and 3. In the first stage or X-ray absorption stage, energy, such as an X-ray, is delivered to the sample material from an external X-ray source as depicted in FIG. 2. The energy absorption or energy increase results in the sample becoming excited at the atomic level as shown in FIG. 3. In the second stage or the X-ray emission stage, after a time interval equal to the life time of the characteristic excited state, the sample spontaneously decays and emits the excess energy in the form of X-rays with energies uniquely related to the electronic structure of the sample species as illustrated in FIG. 3.
The emission spectrum is plotted as X-ray intensity versus energy. The line features of the emitted X-ray spectrum are characteristic for the chemical composition of the sample material and thus can be interpreted as fingerprints in a virtually unequivocal elemental identification. Line intensities are used in quantitative analyses with appropriate calibration. The bremsstrahlung radiation background, always present in the recorded spectrum, can be subtracted by a computer used for data processing.
It would be advantageous to substantially improve the efficiency of early risk detection by incorporating a miniature analytical instrument within the machinery assembly itself to conduct a continuous, real-time oil analysis and detection of metal particles of any size. It would be advantageous to be able to have an X-ray fluorescence spectroscopy that did not have to be run in a batch mode. It would further be advantageous to be able to have immediate detection of the formation and release of wear metal particles.